The present disclosure relates to a measuring device for an optical measuring system, with a rigid body which comprises a probe body or a tool, and with a holding part for holding the measuring device by hand or for clamping the measuring device in a machine, wherein at least one optical marker is arranged on the measuring device and is detectable via a camera in order to be able to determine the position of the measuring device, in particular the position of the probe body or of the tool of the measuring device.
The present disclosure further relates to a measuring system with such a measuring device, a camera for capturing image data of the measuring device, and an evaluation and control unit which is configured to evaluate the image data captured by the camera and, from these data, to determine position data of the measuring device.
An exemplary measuring device and an exemplary measuring system are known from DE 10 2015 205 615 A1.
Measuring systems with measuring devices of this kind are used, for example in the context of quality assurance, to check workpieces or to fully determine the geometry of a workpiece in the context of what is called reverse engineering. Moreover, diverse further application possibilities are conceivable, such as process-controlling applications, in which the measurement technique is applied directly for on-line monitoring and regulation of manufacturing and machining processes. A common application example is that of checking vehicle body components in respect of possible manufacturing faults. In principle, however, such measuring systems can be used to measure any type of measurement objects.
Measuring systems having handheld measuring devices serve as an alternative to more complicated coordinate-measuring appliances in which the workpieces are measured either optically and/or in a tactile manner on a stationary or permanently installed machine with a relatively complex structure.
On account of the mobile usability, measuring systems having handheld measuring devices are becoming increasingly important since they would extend the range of uses yet further in comparison with stationary or permanently installed coordinate-measuring appliances solely on account of their more flexible usability. However, the extremely stringent requirements made in terms of the measurement accuracy that these measuring systems are intended to deliver often militate against the usability of such a mobile measuring system. It is true that manifold digital-optical possibilities now exist, in particular software methods, in order that, from images or films of objects or scenes, the spatial structure of the imaged objects in the scene can be deduced. However, in principle, these methods have some shortcomings which result in them currently not yet being feasible for many highly precise measurements and instead being used only for measurements which have lower requirements in terms of the measurement accuracy.
In the measuring system known from DE 10 2015 205 615 A1, a tactile probe head, which can be used to manually scan a workpiece to be measured, is arranged on a manually portable measuring device. Furthermore, a plurality of optical markers are arranged on the handle of the measuring device and regularly emit infrared beams which are captured from outside using a camera system. The camera images captured by the camera system are evaluated in a computing unit, the position and orientation of the markers in space being calculated by means of a suitable computing algorithm. This is usually carried out using optical triangulation methods. The location and position of the probe head or of the probe body relative to the markers can be determined by means of a calibration step. If a user guides the measuring device by hand towards a workpiece, with the result that the probe body touches the workpiece, a measuring point on the workpiece can thus be determined The shape and location of the workpiece relative to the camera system ultimately result from a suitable multiplicity of such measuring points.
However, the measuring system known from DE 10 2015 205 615 A1 has at least two important disadvantages. On the one hand, active infrared light sources are used as markers. Such active markers which are integrated in the handheld measuring device have the disadvantage that, on account of the development of heat caused by them, they give rise to material expansions which can result in measurement errors. Such measurement errors cannot be disregarded at all in optical measurement technology. On the other hand, in the system known from DE 10 2015 205 615 A1, the user has to manually actuate a button on an actuation unit in order to signal to the computing unit that a measuring point is intended to be captured or in order to store a currently captured measuring point. Since the user inevitably exerts a force for this purpose, the magnitude and direction of which force are unknown, the probe head can be readily deformed, shaken or shifted. This results in measurement errors which cannot be readily compensated.
Similar problems also arise in a system sold by Optinav under the name “OptiTrace” (http://optinav.pl/en/info/products/optitrace.htlm, retrieved on 22 Dec. 2015). Although no active markers are used here on the measuring device, a button on the computing unit or the measuring device also has to be actuated here in order to capture and store a measuring point.
In stationary or permanently installed coordinate-measuring appliances, the abovementioned problem of adopting a measuring point, that is to say capturing and storing the measuring point, is often solved by means of additional sensors which are integrated in the probe head of the coordinate-measuring appliance. One example of such a system is known from WO 2006/114627 A1. In this case, the probe body or the measuring tip is coupled to the quill of the coordinate-measuring appliance via springs of the probe head. The movement of the probe head relative to the quill is determined using a measuring system. Such probe heads are also referred to as passively measuring sensors for coordinate-measuring appliances.
A similar measuring probe having a load sensor, which is integrated therein, measures the force acting between probe body and workpiece and controls the measurement recording on the basis of the signal generated by the load sensor, is known from EP 1 984 695 B1.
Although such sensors could also be used in handheld measuring systems, this would considerably increase the overall complexity of the measuring system. In particular, as a result of the additional sensors, further active components would be accommodated in the measuring system, with the result that temporal synchronization of the sensor signals with the signals from the optical tracking system would be required.
A further example of a handheld coordinate-measuring appliance is known from EP 0 703 517 B1. Apart from the relatively high degree of complexity of this system, the mobile usability is restricted here by the fact that the probe head is connected to a fixed column via a movably mounted carrier.